Environmentalists plan to sue a federal agency next week to release court-sealed documents detailing the activity of endangered condors on the site of the proposed Tejon Ranch project northwest of the Santa Clarita Valley.

The Center for Biological Diversity has accused the U.S. Fish and Wildlife Service and Tejon Ranch of stalling the release of the reports, saying the sealed documents might contain damning information about the planned 23,000-home development’s impact on wildlife.

“We expect documents are going to be released that are going to weigh heavily against (the development),” said Adam Keats, a spokesman for the center. “Tejon Mountain Village (one of three parts of the project), as proposed right now, is a death sentence to condors in the wild.”

The developer, Tejon Ranch Company, denied any wrongdoing and said this week it agrees the court should unseal the documents.

A federal court sealed the documents in the late 1990s when Tejon Ranch Company began hammering out proposed terms for the project with Fish and Wildlife and two environmental non profit groups: the Sierra Club and the National Resources Defense Council.

The groups have settled on a plan that sets aside 90 percent of the 270,000-acre swath of land for conservation.

On the other 10 percent, the developer would build a 1,450-acre business park, a mountain resort and an 11,700-acre master-planned community with 23,000 homes.

Joel Reynolds of the National Resources Defense Council praised the deal.

“Tejon Ranch is the holy grail of conservation in California,” said Reynolds, the group’s urban program director and a participant in the negotiations.

“There aren’t large pieces of land like this available anymore,” Reynolds said. “It’s probably one of the biggest conservation agreements in the history of California.”

About 160 California condors are currently living in the wild, according to most estimates, with the majority living in and around Southern California. It was unclear how many live at least part of their lives on Tejon Ranch Company property.

However, in 2003, a rare wild-born condor was illegally shot dead there.

The property itself is remarkable, Reynolds said, because it is where four wildly different ecosystems converge.

To the northeast of the property, the southern Sierra Nevada looms. The San Joaquin Valley stretches out to the north. To the southwest lies the coastal range, while the Mojave Desert borders the land to the south and east.

“All these ecosystems come together right at the ranch, and it’s an amazing mixture,” he said.

The proposed development, Reynolds said, would sit along the western edge of the property near Interstate 5, leaving the vast majority of the wilderness undisturbed.

Despite his satisfaction with the plan, Reynolds and an official with the Sierra Club sent a letter to Tejon Ranch officials encouraging the firm to request the release of documents related to their negotiations.

Tejon Ranch executive Barry Zoeller said Tuesday the developer agrees now that the plan is finished.

“This will lift (the seal) on all of the documents so everyone can see everything that was underlying or went into the draft plan,” said Zoeller, vice president of corporate communications. “It’s important for appearance’s sake.”